This invention relates to thermal control of processes and, in particular, to high temperature electrochemical converters and associated heat exchange units.
The "traditional" method for fuel cell thermal management is to force a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Cooling water is often employed for ambient temperature devices, and air can be employed for higher temperature fuel cells. In some instances, the same air which serves as the fuel cell's oxidant is used as a cooling medium as well. Typically, the coolant enters the fuel cell assembly at a temperature either at or near the fuel cell operating temperature. The cooling medium passes through the fuel cell and carries off the thermal energy by its sensible heat capacity. The volume flow of coolant required for this method is inversely related to the designed temperature rise of the cooling medium, which is determined either by the limited range of the electrochemical operation of the electrolyte, or in the case of fuel cells with ceramic components, by constraints associated with thermal stress.
The foregoing limitations on the temperature rise of the cooling medium result in coolant flow rates much higher than those required by the electrochemical reaction alone. Since these large flow quantities must be preheated and circulated, a dedicated reactant thermal management subsystem is required. Such thermal management subsystems normally include equipment for regenerative heating, pumping and processing of the excessive coolant flow. These additional components add substantially to the overall cost of the system.
For illustration purposes, consider a regenerative heat exchanger of a type suitable for preheating the fuel cell reactants and operating with a 100.degree. C. temperature difference, and a typical heat transfer rate of 500 Btu/hr-ft.sup.2 (0.13 W/cm.sup.2). Further assuming a 50% cell efficiency with no excess coolant flow, and operating at an ambient pressure, the heat processing or heat transfer surface area of the regenerator would be of the same order of magnitude as the surface area of the fuel cell electrolyte. Considering an excess coolant flow requirement of 10 times the level required for the fuel cell reactant flow, a representative value for conventional approaches, the heat exchanger surface area would be 10 times larger than the active fuel cell surface area. The large size of this heat exchanger makes it difficult to integrate the heat exchanger with electrochemical converters to form a compact and efficient thermal management system.
Thus, there exists a need for better thermal control approaches, especially for use in electrochemical energy systems. In particular, an improved heat exchange system, having the capability of better regulating and maintaining the operating temperature of an electrochemical energy system, would represent a major improvement in the industry.